The present invention relates generally to medical devices and methods. More specifically, the invention relates to improved barriers for use in pressure measurement catheters, and methods for making and using the improved barriers.
Implantable pressure measurement devices such as pressure measurement catheters may be used in a wide variety of contexts to measure many types of pressures in human patient care, veterinary or research environments. Pressure measurement catheters may be used, for example, to measure pressures in a human or animal including endocardial, arterial, venous, cerebral, intraocular, urinary, intrauterine, penile, thoracic cavity and various other vascular or tissue pressures. Measurement of bodily pressures using implantable catheter devices may be equally useful in patient care and research settings. For example, use of pressure measurement catheters in various research settings is described in U.S. Pat. No. 4,846,191 to Brockway et al., the disclosure of which is hereby incorporated by reference.
In one context, pressure measurement catheters are used to measure endocardial pressure, such as left ventricular pressure. One example of such catheters is described in U.S. patent application Ser. No. 10/077,566, filed Feb. 15, 2002, entitled DEVICES, SYSTEMS AND METHODS FOR ENDOCARDIAL PRESSURE MEASUREMENT, the entire contents of which are hereby incorporated by reference. Other patents, such as U.S. Pat. No. 5,810,735 to Halperin et al. and U.S. Pat. No. 5,904,708 to Goedeke, which are hereby incorporated by reference, disclose systems for monitoring internal patient parameters such as right ventricular (RV) pressure. Although much of the following description focuses on using pressure measurement catheters to measure endocardial pressure in a human patient, it should be emphasized that this is only one potential application of pressure measurement catheters. As mentioned above, pressure measurement catheters may be used in a wide variety of body vessels, cavities, tissues and the like, and may be used in human patient care, veterinary or research settings.
Implantable devices used for pressure measurement in a human or animal often include a pressure transmission catheter (PTC). A PTC is typically a hollow tubular catheter having a proximal end, a distal end and a lumen. The proximal end is connected to the implantable device and the distal end is positioned in an area of interest for measuring pressure, such as a within a tissue, vessel or body cavity. Typically, a barrier such as a plug or membrane is disposed within the lumen of the PTC near the distal end. Examples of such devices are described in U.S. Pat. Nos. 4,846,191, 6,033,366 and 6,296,615 to Brockway et al., the disclosures of which are hereby incorporated by reference. In such devices, the PTC is typically filled with fluid along most of its length. The barrier is positioned in the catheter lumen to both retain the fluid within the transmission catheter and to contact bodily fluid, such as blood or other substances, in a vessel or other body structure. The barrier is generally configured to be microscopically deflectable, so that pressure changes in the bodily fluid cause the barrier to deflect very slightly. This mechanical deflection of the barrier is transmitted to the fluid in the PTC lumen and the lumen fluid, in turn, transmits the mechanical deflection to a pressure transducer.
Typically, barriers used in PTCs are made of a substance having at least some properties of a gel, so that they are often referred to as “gel plugs”, “gel barriers” or “barrier plugs.” The material used to make a gel plug is designed to adhere to the inner wall of a PTC lumen to remain in relatively the same position in the lumen during a period of use. One commonly used material for making a gel barrier, for example, is a dimethyl silicone gel.
A manufacturer of an implantable pressure measurement device typically prepares a barrier material and places the material in the lumen of the PTC to form the barrier plug. Often, a protective tip is placed over the end of the catheter to protect the plug during shipping and handling, and the catheter is then provided to a user. At various times, the user may wish to insert additional barrier material into the catheter. For example, occasionally some of the material is inadvertently removed from the end of the catheter when the protective tip is removed, and a user may want to refill the catheter. In a research setting, a user may wish to reuse an implantable pressure measurement device, such as by using the device in multiple animals in succession. Such a user may wish to refill the catheter with barrier material between animals.
One type of barrier material currently used for making barriers in pressure measurement catheters is Dow Q7-2218 gel, made by Dow Coming (Midland, Mich.). Dow Q7-2218 is typically provided as two component parts which must be mixed to form the final gel. Manufacturer's specifications for Dow Q7-2218 suggest mixing the two components in a 1:1 ratio. Such a 1:1 mixture, however, results in a gel that may be appropriate for some applications but is typically not soft enough to allow placement of the gel within a PTC. Therefore, Dow Q7-2218 is typically mixed “off-ratio” to give the gel the desired softness for use in PTCs. In some applications, for example, Dow Q7-2218 is mixed at a ratio of between about 1.65:1 and 1.90:1. Off-ratio mixing, however, has been found to produce a gel that is only slightly cross-linked when made. Such a gel frequently continues to cross link over time, which can be troublesome because the viscosity of the gel increases over time as it continues to cross-link, making placement of the gel in the PTC more difficult and potentially affecting catheter performance. For example, a gel that continues to cross-link may harden within a syringe for injecting the gel into a catheter, typically requiring the use of a power injector rather than a simple handheld syringe. Additionally, such a slightly-cross-linked gel may have a stringy consistency and/or to produce bubbles while hardening into its final form, affecting the performance of the gel barrier.
Although materials such as Dow Q7-2218 gel mixed off-ratio have been used successfully for making barriers in pressure measurement catheter devices, several improvements could be made to such materials. For example, it would be advantageous to have materials which could be made by combining components in a 1:1 ratio while still being sufficiently soft for placement and use in a PTC. Such 1:1 mixing would likely provide several advantages, such as preventing inconsistencies between batches of the material and providing a more stable material. It would also be advantageous to have a fully (or at least substantially) cross-linked barrier material, to enhance stability of the material and reduce “washout” of the material from the catheter during use. (Stability, cross-linking, washout and other terms are described fully below.)
A need exists, therefore, for improved PTC barrier materials that are both (1) sufficiently soft to provide accurate pressure transmission and convenient handling, and (2) fully or substantially cross-linked when made, to provide stability and reduce washout. Ideally, such barrier materials would be convenient to produce and place within a catheter for use. At least some of these objectives will be met by the present invention.